This class represents an explicit kerning node for the typesetter.
From The TeXbook

A kern_node has a width field to specify a (normally negative) amount of
spacing. This spacing correction appears in horizontal lists between
letters like A and V when the font designer said that it looks better to
move them closer together or further apart. A kern node can also appear in
a vertical list, when its `width' denotes additional spacing in the
vertical direction.